This invention generally relates to prerecorded recording media, such as video disks, video tapes and the like, from which the recorded information is copied by a video tape recorder, and to a device for adding a copy-proof interference signal, which will prevent copying from the recorded recording media, to a composite color video signal to be recorded on the media.
Recently, various types of recording/reproducing systems have been proposed for recording and reproducing various information signals at a high density. Magnetic recording/reproducing apparatus, such as a video tape recorder (VTR) utilizing a magnetic tape as a recording medium; a color VTR using so called phase shift method (PS method) as described in detail, for instance, in Japanese Patent Provisional Publication No. 52-48919; a color magnetic video recording/reproducing device using so called phase invert method (PI method) as described in detail, for instance, in Japanese Patent Provisional Publication No. 50-34419; or the like are being popularized as home use magnetic video recording/reproducing devices (which will be referred to as home-use VTRs hereinbelow) throughout the world. As a high density recording/reproducing system using a disk-like recording medium, various systems have been developed; for instance, one in which reproduction is performed magnetically, one in which reproduction is performed optically, one in which reproduction is performed by detecting the variation in electrostatic capacitance, one in which reproduction is performed by using a piezoelectric element and others. Among other things, one in which reproduction is performed optically and one in which reproduction is performed by detecting the variation in electrostatic capacitance have reached a step which is expected to be greatly progressed to practical use.
A problem in connection with the popularization of home-use VTRs is that the recording contents of a recorded recording medium is readily copied or duplicated by a home-use VTR since a reproduced signal from a recorded recording medium, such as a recorded magnetic tape, a video disk or the like, can be easily recorded by a home-use VTR because of the fact that a home-use VTR per se has a recording function.
It is apparent that if the present circumstances allowing free copying, duplicating or stealing of the recording contents of a recorded recording media by home-use VTRs are left as they stand, it will cause serious damages to the production and selling industries, authors, and other relating parties. However, copyright law does not prohibit copying for private use by prescribing as to copying for private use as, "A work which is an objective of copyright, can be copied by a user if he intends to use it personally or at home or in a limited scope similar to such purposes." Therefore, it is inevitable that the recorded contents of recorded magnetic tapes or video disks are stolen on a limited scale. Therefore, it can be readily predicted that the same problem as the fact that authors, musicians, producers, phonograph record industry are influenced seriously by personal steal-copying which has been a problem in connection with phonograph records, will arise in connection with recorded magnetic recording tapes and video disks. Thus it can be predicted that video disk players, which do not have a recording function, cannot compete with home-use VTRs having a recording function, and therefore, popularization and development of video disks will be seriously affected.
In this way, the fact that copies are produced by readily stealing the contents of recorded media by VTRs will seriously damage various relating industries and relating persons. It is well known that an effective countermeasure has been desired, for this reason, for making it impossible to make copies by stealing the contents of recorded recording media.
It is also well known that many proposals have been made hitherto, in order to make it difficult to steal the contents of a recorded medium, from the viewpoint that a normal picture can be obtained when a reproduced signal from a recorded medium is directly applied to a monitor image receiver, and on the other hand, when the reproduced signal from the recorded medium is once recorded by a VTR, a satisfactory reproduced picture will not be imaged on the monitor image receiver with the reproduced signal from the VTR.
It is often experienced, in connection with the information contents, that in the case that an information signal recorded on a recorded medium is a composite color video signal, its value is appreciated only when it is imaged on a monitor image receiver as a color TV image, and if the signal is imaged as a black and white image on a monitor image receiver, it is of no value. In such a case, steal-copying is invalidated by making sure that a clear color TV picture is imaged on a monitor image receiver when the reproduced signal from the recorded medium is directly applied to the monitor image receiver, and on the other hand, when the reproducing signal from the recorded medium is once recorded by a VTR, and is applied to the monitor image receiver after being reproduced, only a black and white picture is imaged on the monitor image receiver. Furthermore, invalidation of steal-copying can be satisfactorily achieved if it is arranged such that when the reproduced signal from the recorded medium is once recorded by a VTR, and is applied to a monitor image receiver after being reproduced, the original normal colored TV picture is not imaged on the monitor image receiver but a color TV picture of scrambled or confused colors is imaged on the monitor image receiver.
In some conventional prerecorded video tapes, the magnitude of the vertical synchronous signal of the composite color video signal has been made relatively small so that vertical synchronization would not be satisfactorily obtained when the reproduced signal of the tape is once recorded by another VTR. Although such conventional method could prevent the contents of video tapes from being copied to other tapes when copying is made by a VTR, the conventional method has suffered from a fatal problem that vertical synchronization is apt to be lost when it is intended to directly reproduce the recorded information by a monitor TV. Furthermore, because of great progress in circuit techniques in VTRs in recent years, most VTRs now on the market are not affected by such a conventional copy-proof technique. Namely, the above-mentioned conventional copy-proof technique is not effective against recent VTRs.